Liquid-crystals are highly anisotropic fluids that exist between the boundaries of the solid and ordinary (i.e., isotropic) liquid phase. The phase is a result of long-range orientational ordering among molecules that occurs within certain ranges of temperatures. The ordering is sufficient to impart some solid like properties on the fluid, but the ordering is usually not strong enough to prevent flow. This dualism of physical properties is expressed in the term liquid-crystal.
Liquid-crystals may be divided into two broad categories according to the principle means of breaking down the complete order of the solid state: (1) Lyotropic liquid-crystals, which are multicomponent mixtures, result from the action of a solvent, and (2) Thermotropic liquid-crystals, which also may be mixtures of compounds, result from the melting of mesogenic solids and, hence, are thermally activated mesophases.
Within each category, three distinctive structural classes of liquid-crystals have been identified. These structures are related to the dimensionality and packing aspects of the residual molecular order.
(1) The nematic phase is the simplest: there is a preferred direction (referred to as the director) for the long axis, but the spatial distribution of the molecules is random, as in an ordinary liquid.
(2) The cholesteric phase is locally nematic in structure, but on a macroscopic scale a "twist" or helical structure is introduced such that the preferred direction rotates right or left as one proceeds along the optical axis.
(3) The smectic phase also has a preferred direction for the long axis, but an additional degree of order is introduced in that the molecules are spatially arranged in parallel layers. The smectic liquid-crystals are distinguished not only by a parallelism of the molecular long axes, but by a layering of the molecular centers of gravity in two-dimensional planes or sheets.
Smectic A phases are the least ordered of all the smectic structures. The molecules are arranged in equally spaced layers and, thus, define a definite repeat distance which may be measured by X-ray diffraction methods. In the smectic A phase, the long axes of the molecules generally are perpendicular to the layer plane. Within each layer, the centers of gravity are randomly dispersed and there is considerable freedom of translational motion and rotation, but the long axes are relatively unrestricted. In other words, the motion within the dimensions of the layer plane is free like a liquid, but in the direction out of the plane it is highly restricted almost like a solid.
The high degree of order found in smectic phases is useful for constructing thick liquid-crystal cells with a high degree of clarity. Even a perfectly formed thin (hundreds of microns) nematic cell would be turbid and scatter light strongly because of thermal fluctuations in molecular orientation. In the smectic phase, however, the orderly layered structure effectively results in a "molecular vice" which firmly clamps all the molecules into place, reducing the directional fluctuations that single molecules can undergo.
Recently, optically clear liquid-crystal cells which are up to one millimeter thick have been proposed using smectic A phase liquid-crystals. This would not be feasible using nematic or smectic C phase liquid-crystals, since thermal oscillations of the defined direction or "director" cause significant light scattering. However, smectic A can be ordered in thick layers since the director lies perpendicular to layers which contain the molecular centers. These materials show a sensitive nonlinear optical response to Q-switched laser pulses of various wave lengths which indicates that they will be useful in applications including optical limiters against Q-switched laser pulses and display systems using laser smectic liquid-crystal light valves.
Practical applications of thick smectic films require liquid crystalline materials which have a wide smectic A temperature range, narrow nematic range, sharp phase transition temperature, photo stability toward laser and solar radiation, and a low response threshold for power limiting.
Unfortunately, commercially available smectic A compounds and compositions contain certain drawbacks. Specifically, the smectic A temperature ranges for these compounds are from a lower limit of about 0.degree.-16.degree. C. to a upper limit of about 40.degree.-59.degree. C. Under severe environmental conditions requiring high performance, a smectic A liquid-crystal composition having a wider temperature range is desirable.
Thus, there continues to be a need for smectic A liquid-crystal compositions for high-performance applications requiring thermal and chemical stability in combination with a high response speed.